Grid controlled magnetrons



Jan. 3, 1961 E. c. DENCH GRID CONTROLLED MAGNETRONS Filed Jan. 29, 1959 ANODE VOLTAGE HEATER VOLTAGE ATTORNEY GRID CONTROLLED MAGNETRONS Edward C. Dench, Needham, Mass, assignor to Raytheon Company, a corporation of Delaware Filed Jan. 29, 1959, Ser. No. 789,915

8 Claims. (Cl. 31530.63)

This invention relates to magnetrons and in particular to grid controlled magnetrons suitable for supplying a high anode current.

A typical magnetron comprises essentially a hollow anode, a cathode for supplying electrons within the anode, and some means for providing a magnetic field, either constant or variable, within the anode and extending transverse to the paths of electrons traveling toward the anode. A less common arrangement is a cathode surrounding an anode. In operation, the anode is maintained at a relatively high positive potential with respect to the cathode, and the crossed electric and magnetic fields cause the electrons emitted from the cathode to traverse curved paths within the anode. The earliest magnetrons had a single, cylindrical anode and were used as low frequency amplifiers. The electric and magnetic fields were adjusted near cut-off condition, in which the majority of the circling electrons just missed the anode. The signal was applied to vary the magnetic field, and the variable anode current yielded the amplified output signal by application thereof to a load. Because it is necessary to vary the magnetic field to control the current, such a device does not provide high anode currents and is useful only at very low frequencies and, even there, substantial input power is required compared with grid controlled tubes.

In an effort to improve operation, in later magnetrons the anode was split into two or larger even number of segments arranged in a circle about the cathode. Such magnetrons are referred to as being of the split-anode type." A different type developed for the same purpose is referred to as the multi-cavity type travelling wave magnetron.

Magnetrons, as heretofore constructed, have included composite cathode structures which utilize the phenomenon of secondary emission for the production of the major part of the anode-cathode electron stream. However, it has been found that such cathodes can, under certain conditions, become overheated and fail due to back-bombardment of the cathode and this may set a limit on the output power obtainable. As used herein, secondary emission means the known effect whereby the bombardment of a surface by an electron stream results in the emission from this surface of further secondary electrons whose number may considerably exceed that of the primary or bombarding electrons. The average number of such secondary electrons released by one primary electron is termed the secondary emission coefficient, a factor which varies with the nature of the emitting surface and the velocity and angle of impingement of the bombarding electrons. It is known that grids have been tried in magnetrons previously, but were considered ineffective and undesirable. This resulted fromthe fact that the grids, in order to produce control of the electron stream, were placed between the cathode and the anode in the electron stream. Such grids proved to be relatively ineffective in controlling the electron 2,967,264 Patented Jan. 3, 1961 2 stream and became rapidly overheated due to bombardment by the electrons.

In view of the disadvantageous effects of back-bombardment and other well-known important considerations, it may now be obvious that the prior art teaches that a grid located between the cathode and anode of a magnetron is not desirable and that use of such a device will provide no advantageous result.

Because of the inability to provide a magnetron suitable for operation as a diode it has been the practice heretofore to use thyratrons or the like for trigger relays and modulation of the output signal of radar transmitters. However, such an arrangement is subject to the serious disadvantage that, inter alia, switching time is limited and cannot be reduced below a specific point determined by the characteristics of the tube itself. Further, the life of such tubes is relatively short due, for example, to back-bombardment and the tendency of the gas in the tube to clean up after the tube has been in use for some time. Substantial effort has been directed to extending the life of such tubes without substantial success.

These and other disadvantages are overcome by the present invention which is concerned with high transconductance grid controlled tubes using magnetron geometry for supplying high anode currents. in accordance with one feature of the invention a cathode structure is concentrically positioned within and along the axis of the central cavity of a cylindrical metal anode block constructed in accordance with magnetron geometry and the provision of an axial magnetic field. The cathode structure is comprised of a substantially cylindrical and coaxial member which functions as a secondarily emissive cold cathode and a heatable member having an area substantially less than that of the cold cathode functions as a thermionically emissive hot cathode which is located at or adjacent the periphery of the cold cathode whereby electrons emitted by the thermionic emitter may, when a magnetic field is applied parallel to the axis of the anode, strike the secondarily emissive cathode to cause secondary emission and thereby produce a high anode current. Additionally included is a radially disposed barrier or collector plate integral with the anode and positioned such that its innermost end is near the thermionic emitter. The collector plate functions to provide what is generally referred to as a non-reentrant tube and especial y to prevent back-bombardment of a grid associated with the thermionic emitter. The aforementioned grid structure is disposed between the anode and the thermionic emitter and adapted to substantially cover the thermionic emitter and functions to provide accurate control of the start of anode current by determining the time at which the electrons emitted by the thermionic emitter are able to escape through the grid and strike the exposed secondarily emissive cathode to provide a high anode current.

In a tube utilizing the present invention, the grid, in combination with a suitably formed trigger pulse applied thereto, functions to provide means for accurately determining the time at which high anode current flow will begin as distinguished from the unsuccessful attempts of the prior art to control anode current with a grid after the anode current has been established; and the collector plate functions primarily to prevent destruction of the grid due to back-bombardment as distinguished from prior use of collector plates to prevent the occurrence of oscillatory currents.

The features of the invention which are believed to be novel are set forth with particularities in the appended claims. The invention, itself, both to its organization and method of operation, together with further objects and advantages may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

Fig. 1 shows in cross section a plan view of a grid controlled tube utilizing magnetron geometry constructed in accordance with the present invention; and

Fig. 2- is a perspective view of a portion of the secondarily emissive. cathode showing the relation of the, secondarily emissive cathode, thermionic emitter and grid structure.

Referring now to Fig. 1, there is shown an electron discharge device 111 of the magnetron type having therein a specific illustrative embodiment of the present invention. A cylindrical envelope 11 contains therein an anode block 12 having a cylindrical central cavity 13, having a smooth inner surface 14 and a cathode structure 15 which comprises a hollow cylindrical non-thermionic or cold cathode 16 located along the axis of the magnetron and having a slot 17 to receive a thermionic, or hot, electron emissive cathode 18. A suitable anode voltage source 8 is connected between the anode 12 and cathode 16. The emissive cathode 18 may be indirectly heated, but it is shown in Figs. 1 and 2 as a directly heated filament wire 18 which is longitudinally disposed in the slot 17. One end 19 of the emissive cathode 18 is connected to the cold cathode 16 and the other passes through and is movably supported by a nonconductive support 21). Lead 21 is connected to the emissive cathode 18 and is adapted for connection to a suitable heater voltage source 9 that may be connected between lead 21 and cathode 16.

The cold cathode 16 may be made of a material which will emit sufficient electrons when it is bombarded with a small number of primary electrons, i.e., electrons from the emissive cathode 18, to generate the power output for which the device 119 is designed. The cold cathode 16 may advantageously be made of beryllium oxide on copper which is a good secondarily emissive material, i.e., one in which an impinging electron causes a plurality of electrons to be emitted.

The most efficient and suitable means for providing a source of primary electrons is the use of thermionic emission. Such a thermionic emitter may comprise a simple tungsten filament which may if desired be coated with a suitable emitting material. If preferred, an indirectly heated element may be suitably coated with an emitting material so that it will operate at a lower temperature of, for example, 850 C. and located at a distance of about 1 mm. from the cold cathode 16. Present-day emitters will provide an adequate supply of primary electrons to provide a relatively heavy secondary emission current from a cold cathode even though a comparatively poor secondary emitter is used. For the purposes of the present invention it is preferable that the area of the emitting surface of the thermionic emitter be as small as possible to prevent it from becoming overheated in operation.

Disposed over and covering the thermionic emitter or hot cathode 18 is a grid structure 22 shown, by way of example, in Fig. 2 as a wire grid adjacent to and covering the thermionic emitter 18 such that when a suitable potential (not shown) is connected to the lead 23 the electrostatic charge on the grid 22 will prevent the escape of primary electrons from the thermionic emitter 18 into the portion of the cavity defined by the inner surface 14 of the anode 12 and the outer surface 24 of the cold cathode 16. To prevent back-bombardment and overheating of the grid 22, a radially disposed collector plate 25 is positioned at or near one axial edge of the grid 2.2 in a plane transverse to the inner wall 14 of the anode 12. One end of the collector plate 25 is coextensive with the inner surface of the anode 12 and extends radially inward therefrom to a point adjacent the surface 24 of the cold cathode 16 and near an edge of the grid structure 22.

A tube incorporating the present invention operates in the following manner: Production of the anode current is eifected by bombardment of the secondary emissive surface 24 by means of the magnetic field which is normally utilized for the operation of a magnetron, the direction of this field being substantially at right angles to the direction at any point of the electrostatic field between the anode and the cathode.

It is known that an electron moving in a vacuum and acted on solely by a uniform magnetic field whose direction is at right angles to its plane of motion will theoretically traverse a circular path whose radius is directly proportional to the velocity of the electron; if an electric field is also present, however, the path will take a generally curved form whose exact shape depends on the relative strengths and directions of the two fields.

Considering now the free electrons initially produced in the apparatus according to the invention by any of the means previously described, it will be seen that under the action of the applied electrostatic field between the anode and cathode such electronswill commence to move towards the anode; the presence of the magnetic field will, however, cause them to follow curved paths, and some of them will thus return and strike the cathode. Secondary electrons will thus be released, and of these a certain number will, under the action of the magnetic field, traverse paths whose curvature is sufficiently great to cause them to return to the cathode. These in turn will cause the release of further secondary electrons, and the secondary emission will thus build up automatically and very quickly until an equilibrium condition, dependent upon the space charge distribution, is reached. Of the total number of electrons emitted by the cathode a certain percentage thus return to maintain the process of secondary emission, while the remainder constitute the main electron stream required to carry out the function of the apparatus. As indicated hereinbefore the aforementioned magnetic field may be provided by any suitable permanent magnet, electromagnet or solenoid arranged either externally or internally with respect to the discharge apparatus.

In order to control accurately the instant of conduction and/or cutoff of the tube, the grid 22 is included to supply a suitable electrostatic field, provided by a square wave trigger pulse, for example, to suppress the primary electrons emitted by the thermionic emitter 18 and prevent the primary electrons from entering the anode-cathode space until the instant that it is desired that the tube begin conduction. At the instant of removal of the electrostatic field at the grid 22 primary electrons emitted by the thermionic emitter 18 will enter the anode-cathode space and substantially simultaneously by reason of secondary emission supply a large anode-cathode current as and for the purposes described hereinbefore. During conduction the collector plate 25 performs the principal function of preventing secondary electrons from striking the grid 22 and the thermionic emitter 18 and causing them to become overheated, ineffective or damaged.

As a result of the novel structure of the present invention there is provided a high transconductance and grid control tube using magnetron geometry that will supply a high anode current and that is particularly useful as a trigger relay for replacing the functions of thyratrons and ignitrons.

Additionally, the present invention makes possible a superior diode magnetron that is simple and therefore dependable, that is economical to manufacture and that is able to produce a high anode current at precise intervals of time in response to predetermined trigger pulses.

While the present invention has been described in its preferred embodiment, it is realized that modifications may be made and it is desired that it be understood that no limitations on the invention are intended other than may be imposed by the scope of the appended claims. For example, the relative positions of the anode and thiscathode may be reversed without deviating from the I present invention.

What is claimed is:

1. In an electron discharge device of the magnetron type the combination comprising: an anode block defining a central cavity; a substantially cylindrical secondary emission surface coaxial with the central cavity; a thermionic emitter located adjacent said surface for providing primary electrons to bombard said surface; and means located between said anode and said emitter adjacent and covering said thermionic emitter for regulating the escape of said primary electrons to excite secondary emission from said surface.

2. In an electron discharge device of the magnetron type the combination comprising: an anode block defining a central cavity; a substantially cylindrical secondary emission surface coaxial with the central cavity; a thermionic emitter having an area substantially less than that of said surface and located adjacent thereto for providing primary electrons to bombard said surface; and a grid structure located between said anode and said emitter adjacent and covering said emitter for regulating the escape of said primary electrons to excite secondary emission from said surface.

3. In an electron discharge device of the magnetron type the combination comprising: an anode block defining a central cavity; a substantially cylindrical secondary emission surface coaxial with the central cavity; a thermionic emitter having an area substantially less than that of said surface and located adjacent thereto for providing primary electrons to bombard said surface; a grid structure located adjacent and covering said emitter for regulating the escape of said primary electrons to excite secondary emission from said surface; and means for rendering said device non-reentrant.

4. In an electron discharge device of the magnetron type the combination comprising: an anode block defining a central cavity; a substantially cylindrical secondary emission surface coaxial with the central cavity; a thermionic emitter having an area substantially less than that of said surface and located adjacent thereto for providing primary electrons to bombard saidsurface; a grid structure located adjacent and over said emitter for regulating the escape of said primary electrons to excite secondary emission from said surface; and means for rendering said device non-reentrant, said means comprising a radially disposed collector plate having one end integral with said anode and its opposite end adjacent said surface.

5. In an electron discharge device of the magnetron type the combination comprising: an anode block defining a cylindrical central cavity; a substantially cylindrical secondary emission surface coaxial with the central cavity; a thermionic emitter comprising a filament wire located adjacent said surface for providing primary electrons to bombard said surface; a grid structure located adjacent and over said emitter for controllably preventing the escape of said primary electrons to excite secondary emission from said surface; means for establishing a magnetic field coaxial with said cavity; means for connecting a source of current to said wire to heat said wire to electron emission temperature; means for connecting a potential between said anode and said surface whereby said anode is rendered positive; means for connecting a potential to said grid for determining the time at which electrons are able to escape through said grid; and means for substantially preventing back-bombardment of said grid comprising a radially disposed collector plate having one end integral with said anode and its opposite end adjacent 6 said surface and in proximity with an edge of said grid structure.

6. An electron discharge device of the magnetron type comprising: an anode block defining a cylindrical central cavity having a smooth inner surface; a substantially cylindrical secondary emission surface coaxial as well as coextensive with the central cavity; a thermionic emitter comprising a filament located adjacent said emission surface for providing primary electrons to bombard said emission surface; a grid structure located adjacent and over said emitter for controllably preventing the escape of said primary electrons into said central cavity; means for establishing a magnetic field in said cavity; means for connecting a source of current to said wire to heat said wire to electron emission temperature; means for connecting a potential between said anode and said emission surface whereby said anode is rendered positive; means for connecting a potential to said grid for determining the time at which electrons are able to escape through said grid and excite secondary emission from said emission surface; and means for substantially preventing backbombardment of said grid comprising a radially disposed collector plate coextensive with said cavity and having one end integral with said anode and its opposite end adjacent said emission surface and near an edge of said grid structure.

7. An electron discharge device of the magnetron type for providing a high anode current comprising: an anode 'block defining a cylindrical central cavity having a smooth inner surface; a substantially cylindrical secondary emission surface coaxial as well as coextensive with the central cavity; a thermionic emitter comprising a filament wire located adjacent said surface for providing primary electrons to bombard said emission surface; a grid structure located between said anode and said emitter and adjacent and over said emitter for regulating the escape of said primary electrons to excite secondary emission from said emission surface; means for establishing a magnetic field coaxial with said cavity; means for connecting a source of current to said Wire to heat said wire to electron emission temperature; means for connecting a potential between said anode and said emission surface whereby said anode is rendered positive; means for connecting a trigger pulse to said grid for accurately determining the time at which electrons are able to escape through said grid and cause the flow of a high anode current; and means for substantially preventing back-bombardment of said grid comprising a radially disposed collector plate coextensive with said cavity and having one end integral with said anode and its opposite end adjacent said emission surface and near an edge of said grid structure.

8. In an electron discharge device of the magnetron type the combination comprising: a substantially cylindrical anode; a substantially cylindrical secondary emission surface coaxial with said anode; a thermionic emitter located adjacent said surface for providing primary electrons to bombard said surface; and means located between said anode and said emitter adjacent and covering said thermionic emitter for regulating the escape of said primary electrons to excite secondary emission from said surface.

References Cited in the file of this patent UNITED STATES PATENTS 2,409,038 Hansell Oct. 8, 1946 2,508,280 Ludi May 16, 1950 2,760,111 Kumpfer Aug. 21, 1956 

